Optoelectronic distance measuring devices of the generic type are sufficiently well known from the prior art. They have a distance measuring range of a few tens of meters and are often in the form of hand-held devices. They are used mainly in construction surveying or in interior finishing, for example for three-dimensional surveying of rooms. Further fields of use for distance measuring devices are geodetic and industrial surveying. The basic principle of distance measurement with the known devices is based on the evaluation of a change, as a function of time, of a characteristic of the electromagnetic radiation emitted by the device and reflected by an object sighted. The distance measuring device is equipped for this purpose with a transmitter for emitting intensity-modulated radiation. Hand-held devices chiefly employ optical radiation in the visible wavelength spectrum in order to facilitate the sighting of the measuring points. The optical radiation is reflected by the measured object sighted and detected by a receiver installed in the device. The distance to the measured object is obtained from the time lag of the received radiation compared with the radiation emitted by the transmitter.
Pin photodiodes or avalanche photodiodes for transforming the received radiation reflected by the measured object into electrical signals are usually used as detectors in the known distance measuring devices. Distance measuring devices whose distance determination is based on the phase measurement principle are very common. In such devices, the electrical received signal is superposed directly on the avalanche photodiode or after preamplification with a mixer frequency to give a low-frequency measuring signal. On this low-frequency signal, the phase is determined and is compared with the phase of a reference signal. The difference between the measured phase of the low-frequency measuring signal and the phase of the reference signal is a measure of the distance to the measured object.
EP-B-0 738 899 describes the behaviour of laser diodes for visible radiation and the associated accuracy problems in distance measurement. For improving the accuracy of the distance measurement, it is proposed there to modulate the emitted laser radiation with pulse widths of less than 2 ns. The modulation frequency of this known device is in the region of about 50 MHz. In the case of pulses having a pulse width of, for example, 1 ns and a period of 20 ns, a pulse power of about 20 mW is required in the case of these known devices in order to achieve an average power of 1 mW which generally ensures sufficient visual certainty. The proposed type of modulation can also be implemented with commercially available 3 mW lasers without having to accept relatively great sacrifices in the lifetime of the laser owing to the increased pulse power compared with continuous 3 mW operation. As a result of the short pulses and the high pulse power, a short coherence length of the laser radiation is achieved. This results in a reduction of the generally granulated intensity distribution of the radiation reflected by the generally rough surface of the measured object sighted. The granulated intensity distribution is also known by the name speckles and influences the achievable accuracy of measurement.
WO 02/16964 describes a method and a device for distance measurement which are based on the phase measurement of optical measuring radiation reflected by a measured object sighted. Intensity-modulated, optical measuring radiation emitted by a measuring device is transmitted to the measured object and a part of the measuring radiation which is reflected by the measured object is detected by a receiver arranged in the measuring device and converted into electrical measuring signals. The electrical measuring signals are then compared with a reference signal which is generated from the detection and conversion of a measuring light component passed through a known reference distance, in order to determine the distance between the measuring device and the measured object from a phase difference. It is proposed to emit the measuring radiation with burst modulation and to evaluate the measuring signal of the receiver only during a timespan dependent on an active burst duration.
The active burst time is that duration during which a burst signal is present, whereas no signal is present at the laser diode as a transmitter during a dead time. The period of the sequence of bursts and dead time is referred to as burst period. The burst signal has a duty cycle which is defined as the ratio of the active burst time to the burst period in %. Thus, the burst modulation differs from a pulse modulation in which the modulation signal is present quasi-continuously over a total duration of a measuring period. In the case of burst modulation, on the other hand, the modulation signal is present only during a part of the measuring period, so that a pulse sequence is emitted only during the active burst time. According to the abovementioned definition, the duty cycle is always 100% in the case of pulse modulation while the value is always less than 100% in the case of burst modulation. The burst modulation can be effected, for example, by means of a burst signal with square-wave modulation.
By evaluating the measuring signal of the receiver only during the active burst duration, the signal/noise ratio (S/N) can be improved. This can be explained by a simplified example of a laser with a maximum average output power of 1 mW. If, instead of the measuring radiation with 2.5 mW peak power emitted in the case of the known devices, a laser burst of 10% duty cycle with a peak power of 25 mW is radiated, an average laser power of 1 mW is obtained again. Because the received signal is evaluated only during the active burst duration, the same total signal which would arise if a continuous signal were summed is obtained. However, since no evaluation takes place during 90% of the period, 90% of the noise may also be suppressed. In this simplified example, this results in an improvement of the signal/noise ratio (S/N) by a factor of √(10), i.e. square root (10).
The burst modulation can be effected in principle with an active burst duration which is limited only to a single peak. Expediently, however, the active burst duration is chosen so that a duty cycle which is about 5% to about 50%, preferably about 10% to about 40%, results therefrom.
For the burst modulation effect, the emitted measuring radiation can be modulated in particular with a modulation frequency of greater than 100 MHz and a peak power greater than 10 mW. The higher peak powers of the laser in the case of burst modulation also shorten the coherence length of the emitted laser radiation since the laser jumps through several modes with the same pulse width but higher peak power. This can have an advantageous effect on the accuracy of the measuring devices.
Furthermore, the burst modulation may also lead to a simplification of the regulation of the laser power and permit a reduction of power consumption.
The distance measuring devices described in WO 02/16964 have a transmitter for emitting burst-modulated optical radiation, a receiving optical system for a part of the optical measuring radiation which is reflected by the measured object, a receiver downstream of the receiving optical system and intended for converting the optical radiation into electrical measuring signals, a device for producing reference radiation, which can be converted into electrical reference signals after passing through a known reference distance, a filter device for filtering out noise signals and a signal processing unit, in particular a digital signal processing unit, for analyzing the measuring signals and the reference signals with regard to their phase position—in order to determine therefrom the distance to the measured object and to make the result available to the user. The transmitter is connected to a frequency synthesizer, with which an intensity modulation based on the burst modulation principle can be impressed upon the emitted optical radiation. On the receiver side, the evaluation of the electrical measuring and reference signals is coupled to the active burst time.
The optical reference radiation can be produced, for example, by a beam splitter and, after passing through a known reference distance, detected by a separate reference receiver and converted into electrical reference signals. The emitted optical measuring radiation can, however, also be passed periodically either to the measured object or through the reference distance to the receiver. For example, a deflection mirror pivotable periodically into the beam path can be provided for this purpose.
For the burst modulation, a semiconductor laser diode for visible optical radiation, which has, for example, a wavelength in the range from about 630 nm to about 650 nm, can be used as a transmitter. Such semiconductor laser diodes can be operated with the required average output powers and can provide in particular the required pulse energies virtually without sacrifices in terms of lifetime.
In the method described in WO 02/16964, the electrical measuring signals are converted by continuous or burst-like superposition of a high-frequency mixer frequency into low-frequency signals and filtered only during the active burst time or converted into an output voltage by a transimpedance amplifier acting as a filter, so that the low-frequency signals can be further processed in a signal processing unit determining the distance to the measured object from the respective phase positions. The mixer frequency may correspond, for example, to the value of the modulation frequency of the burst signals±the frequency value of the low-frequency signal. The filtering out of noise can be effected, for example, on the analogue low-frequency signal and/or carried out after digitizing of the signal in the digital signal processing.
According to WO 02/16964, the active burst time is advantageously chosen as about one and a half periods of the low-frequency measuring signal. The first third of the active burst time is required in order to enable the filter to synchronize. The signal is then summed only during the following two thirds of the active burst time, which corresponds to a complete period of the low-frequency measuring signal.
However, the relatively long duration which is required for a synchronization of the filter proves to be disadvantageous since—in contrast to the actual idea of the burst modulation principle—a relatively long active burst time therefore also has to be chosen. The advantages actually achievable by the burst modulation—in particular with regard to an improvement in the signal/noise ratio (S/N)—are thus realizable only to a limited extent by the distance measuring device described in WO 02/16964.